FIELD OF THE INVENTION The invention relates to a process for producing a printing form.
In the production of screen-printing stencils, it is known to apply a photosensitive resist (lacquer) to a screen serving as the carrier of the printing form or stencil, to expose it and subsequently to develop it, in order to remove the photoresist layer in regions in which later, during printing, ink is intended to be transferred through the screen. The exposure, as conditioned by the pattern, of the photoresist layer is in this case performed, for example, by means of a large film laid onto the printing form blank or point by point with the aid of laser light, the entire photoresist layer being scanned point by point and exposed only at those locations at which the chemical structure of the photoresist is intended to be changed, as conditioned by the pattern, for the later development. Furthermore, it is known to vaporize the photoresist layer away, in accordance with the pattern, using a CO.sub.2 laser.
Both the use of large films and the use of lasers, in particular CO.sub.2 lasers, which have a very high energy demand, makes the production of screen-printing stencils more expensive.
In the production of a flexographic printing form, a radiation-sensitive or photosensitive material, for example a photopolymer or photoelastomer is first applied to a carrier. Then, by means of irradiating or exposing individual regions of the layer, as conditioned by the pattern, the polymer is crosslinked. Ultraviolet radiation is preferably used for the exposure. Subsequently, in a suitable developer, the unexposed and hence noncrosslinked polymer regions are washed out. If appropriate, using a radiation source emitting in the UV range, the photopolymer layer is re-exposed and hence more intensively crosslinked.
In the case of the previously known methods, the regions of the radiation-sensitive layer that were not to be exposed in accordance with the pattern, that is to say in particular of the photoelastomer layer, are covered, either by a full-size film, as in the case of screen-printing stencils, or by means of a layer applied in the shape of the pattern by means of nozzles.
In the case of covering the radiation-sensitive layer with a photographically produced full-size film, a seam in the pattern in the case of cylindrical printing forms is generally impossible to avoid. In any case, such a full-size film usually which must have the size of the printing forme. Therefore, in the case of cylindrical printing formes the size of the cylinder surface, makes the production of printing formes considerably more expensive.
The spraying on process, in accordance with the pattern, of a covering, light-absorbing and/or light-reflecting ink layer with the aid of nozzles onto a radiation-sensitive layer places demands on highly-pure covering inks, which adhere adequately to the radiation-sensitive layer and nevertheless do not block up the nozzles used. In addition to the high requirements on the covering inks, the nozzles used for the spraying on process must also satisfy high requirements, since a large number of droplets of constant size have to be produced per unit time.
Although the problems which occur during the handling of large films may be avoided by spraying covering ink in accordance with the pattern onto the surface of a printing forme, in particular onto a cylinder surface, the spraying on technique requires very precise nozzles, which are complicated to produce and to control.